Zinc limitation triggers anticipatory adaptations in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) has complex and dynamic interactions with the human host, and subpopulations of Mtb that emerge during infection can influence disease outcomes. This study implicates zinc ion (Zn2+) availability as a likely driver of bacterial phenotypic heterogeneity in vivo. Zn2+ sequestration is part of nutritional immunity, where the immune system limits micronutrients to control pathogen growth, but this defense mechanism seems to be ineffective in controlling Mtb infection. Nonetheless, Zn2+-limitation is an environmental cue sensed by Mtb, as calprotectin triggers the zinc uptake regulator (Zur) regulon response in vitro and co-localizes with Zn2+-limited Mtb in vivo. Prolonged Zn2+ limitation leads to numerous physiological changes in vitro, including differential expression of certain antigens, alterations in lipid metabolism and distinct cell surface morphology. Furthermore, Mtb enduring limited Zn2+ employ defensive measures to fight oxidative stress, by increasing expression of proteins involved in DNA repair and antioxidant activity, including well described virulence factors KatG and AhpC, along with altered utilization of redox cofactors. Here, we propose a model in which prolonged Zn2+ limitation defines a population of Mtb with anticipatory adaptations against impending immune attack, based on the evidence that Zn2+-limited Mtb are more resistant to oxidative stress and exhibit increased survival and induce more severe pulmonary granulomas in mice. Considering that extracellular Mtb may transit through the Zn2+-limited caseum before infecting naive immune cells or upon host-to-host transmission, the resulting phenotypic heterogeneity driven by varied Zn2+ availability likely plays a key role during early interactions with host cells. Author summaryMycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), has plagued humanity for millennia and remains the world's deadliest bacterium today. Bacterial heterogeneity is one of the most important characteristics of Mtb that complicates TB treatment. Access to zinc ion (Zn2+) may influence bacterial heterogeneity, considering microenvironments developed during TB create a perpetual cycle exposing Mtb to high and low concentrations of Zn2+. Here we show that Zn2+ limitation drives changes in gene expression patterns of well described virulence factors in Mtb, and Zn2+-limited Mtb show increased resistance to oxidative stress and increased replication in vivo. Our results suggest that host-pathogen interactions are influenced by pre-exposure of Mtb to Zn2+ and mycobacteria that transit through a Zn2+-depleted microenvironment are primed to withstand impending oxidative stress upon subsequent contact with immune cells in the same, or a naive host. Considering that the standard mycobacterial media recapitulates a Zn2+-replete environment, the Zn2+-dependent phenotype of the pathogen may confound our fundamental understanding of initial interactions between Mtb and immune cells.

Automated summary

This study suggests that zinc ion availability may influence the bacterial heterogeneity of Mycobacterium tuberculosis, leading to changes in gene expression patterns and increased resistance to oxidative stress in vivo. This could have implications for host-pathogen interactions and the ability of Mtb to adapt to different environmental conditions.